Microsoft's Norton AV killer, OneCare is XP only. Makes sense for Microsoft. Once Norton abandons the AV business, as they will, anyone not using Win2K or Win98 will need to switch.
A detail nobody seems to be mentioning.
Microsoft could give this sucker away and still make a vast fortune on the upgrade licenses. They only charge for it because otherwise they'd lose an antitrust battle.
Since OneCare can be installed on 3 machines for $50, and NAV lists as $70/machine, it's much cheaper than NAV. If it inflicts less of a performance hit than NAV it'll be much better too.